1. Field of Application
The present invention relates to a vehicle-mounted radar apparatus for detecting objects such as preceding vehicles, by transmitting and receiving radar waves such as millimeter-range radio waves.
2. Description of Prior Art
In the prior art, types of vehicle-mounted radar apparatus are known which are utilized as part of a vehicle control system such as a cruise control system) for collision prevention or for implementing a xe2x80x9cfollowingxe2x80x9d function whereby a host vehicle equipped with the radar apparatus is controlled to follow an immediately preceding vehicle with a specific separation distance from that vehicle.
It is basically essential for a vehicle-mounted radar apparatus to be capable of detecting a target object such as a preceding vehicle which is directly in the vehicle lane of the host vehicle or which is moving in an adjacent vehicle lane but partially protrudes into the vehicle lane of the host vehicle, or which is in the process of xe2x80x9ccutting inxe2x80x9d ahead of the host vehicle (i.e., is moving from an adjacent vehicle lane into that of the host vehicle). To achieve such detection, it is necessary for the radar apparatus to be capable of substantially accurately determining the current lateral position of a target object and also the successive lateral positions which have been attained by that target object up to the current time point. The term xe2x80x9clateral positionxe2x80x9d of a target object as used herein signifies the lateral position of a width-center location on the target object in relation to a corresponding location on the host vehicle (i.e., lateral with respect to the direction of motion of the host vehicle). The term xe2x80x9cwidth-center locationxe2x80x9d signifies a location midway between left and right sides of an object, such as a location midway between the opposing sides of a vehicle.
In the following it will be generally assumed that the radar apparatus is of FMCW (Frequency Modulation Continuous Wave) millimeter-wave type, although the principles of the invention are not limited to such a type. Each interval in which a transmitting/receiving operation of the radar apparatus is executed, with reflected waves being thereby received from one or more target objects and processing of resultant received signals then performed, will be referred to as a modulation interval.
As illustrated in the conceptual diagram of FIG. 18A, when a radar apparatus of a host vehicle 41 travelling along a straight path transmits radio waves along the direction indicated by the arrow line, the waves will be reflected from various different parts of a target object positioned directly ahead of the host vehicle, with the target object assumed here to be a preceding vehicle 50. Locations from which the waves may be strongly reflected from the rear end of the preceding vehicle 50 are indicated by the black dots 60. In general the waves will be most strongly reflected from various different parts of the target object, in successive modulation intervals. The received signals resulting from the reflected waves, in a modulation interval, are processed to obtain an estimate of the lateral position of the target object. However the obtained position will be determined by the locations of those portions of the target object from which the strongest reflections occur and so will not necessarily coincide with a width-center location on the preceding vehicle, and these portions from which the strongest reflections occur will change with time (for example, due to variations in the attitude of the target object with respect to the host vehicle).
As a result, when successive estimated lateral positions of a preceding vehicle are derived based upon such received radio waves, these will deviate from the actual lateral positions, with the amount of deviation varying with time. This is illustrated in the example of FIG. 18C, in which the curve xe2x80x9cmomentary position dataxe2x80x9d represents a series of estimated lateral position values for a target object such as the preceding vehicle 50, obtained at respective successive modulation intervals. The curve designated xe2x80x9cfinal lateral position dataxe2x80x9d express a corresponding series of successive estimated lateral positions for that target object which have been obtained by smoothing processing (e.g., low-pass filtering) of the momentary position data. The chain-line curve indicates the corresponding series of actual lateral positions of the target object, i.e., of the width-center location of the target object.
The aforementioned variations in the locations on a preceding vehicle from which the radio waves are reflected back to the radar apparatus are determined by factors such as shapes of the portions from which reflections occur, the materials constituting these portions, undulations in the road surface which affect the attitudes of the vehicles, etc. The strongest reflections will typically occur for example from the rear fender, rear reflector plates, the number plate, rear windshield, etc., of a preceding vehicle. As a result, in many cases, the variations in the momentary position values value may be much more extreme and irregular than for the case illustrated in FIG. 18C. In that case, the final lateral position data which are obtained by smoothing the momentary position data will be unstable, and will deviate substantially from the successive lateral positions attained by the width-center location of the target object.
Such data are therefore not suitable for use by a vehicle control apparatus such as a cruise control apparatus, as a basis for automatic control of the host vehicle.
Furthermore as illustrated in the example of FIG. 18B, the host vehicle 41 may be moving along a vehicle lane 42 which is curved, in which case the orientation of a preceding vehicle will become skewed with respect to the host vehicle. As a result, an immediately preceding vehicle (i.e., which is travelling along the same vehicle lane as the host vehicle) will not be located directly ahead of the host vehicle, and reflected radio waves may be received from a side face of that preceding vehicle. Similarly, when a vehicle is driving in a vehicle lane which is adjacent to that of the host vehicle, such as the preceding vehicle 51 shown in FIG. 18B, then such side reflection waves may also occur. This further increases the amount of error which will be arise in lateral position values which are obtained by simply applying smoothing to the series of momentary position values.
More specifically, with the example of FIG. 18B, radio waves will be strongly reflected from the left side face of the preceding vehicle 51 and from portions of the rear end of that vehicle which are closest to the host vehicle 41. For example, the arrow lines designated P1, P2 in FIG. 18B represent peak levels of reflection, which occur at respectively different times, resulting in corresponding local extreme values of the momentary position data oriented in the leftward lateral direction, as illustrated in FIG. 18D. However a peak-level reflection P3 from the right side of the preceding vehicle 51, i.e., from a part of that vehicle which is farther from the host vehicle than the left-side parts of vehicle 51, results in a substantially smaller local extreme value of the momentary position data, corresponding to the rightward lateral direction.
In such a case, as illustrated in FIG. 18B, if the final lateral position data are simply obtained by smoothing the momentary position data, then the resultant data will not accurately represent the successive lateral positions of the width-center location of such a target object, but will strongly deviate towards the left side of the object (in the graphs of FIGS. 17A, 17B, etc., the downward direction from the central axis of each graph corresponds to the leftward direction of position displacement, and the upward direction corresponds to the rightward direction of position displacement).
As a result of such errors in the lateral position data, it may be impossible to accurately judge whether a preceding vehicle is actually moving along the same vehicle lane as the host vehicle. Hence, it is not possible to safely use such lateral position data in a vehicle control system such as a cruise control system, for effecting automatic control of a host vehicle. This is a basic problem of the prior art.
It is an objective of the present invention to overcome the above problem by providing a vehicle-mounted radar apparatus for detecting radio waves received from a target object, whereby the accuracy of determining the lateral position of a target object can be substantially improved.
To achieve the above objective, according to a first aspect, the invention provides a vehicle-mounted radar apparatus including momentary position data generating means which periodically processes received signals from reflected radar waves to derive successive estimated momentary lateral position values for a target object (referred to herein simply as momentary position values, with the series of momentary position values obtained up to the current time point referred to as the momentary position data for that target object) and means for smoothing the momentary position values to obtain final lateral position data, i.e., consisting of a series of lateral position values expressing the approximate lateral positions attained by a width-center location on the target object at successive time points, up to the current point. A vehicle-mounted radar apparatus according to the present invention is characterized by including means for deriving from the momentary position data a series of local extreme values of lateral displacement in one direction with respect to the (momentary) direction of travel of the host vehicle (e.g., the rightward lateral direction), referred to herein as local maximum values of the momentary position data, and a series of local extreme values of lateral displacement in the opposite direction (e.g., the leftward direction) referred to herein as local minimum values of the momentary position data.
If the degree of scattering of the momentary position data exceeds a predetermined level, then a series of local extreme values of lateral position values in leftward direction and a series of local extreme values in the rightward direction, in the momentary position data, are respectively smoothed, and averaging of the results is performed to obtain a single series of corrected position values. These corrected position values are then smoothed, in place of the series of momentary position values, to obtain the final lateral position data for the target object.
As described above, a radar apparatus typically estimates the momentary lateral position of a target object as that of a location, on the object, from which radio waves are most strongly reflected at that moment. Thus, the momentary position values may vary in an irregular manner between positions corresponding to the left side of the target object and positions corresponding to the right side, with the difference between the extreme values of the momentary position being approximately identical to the width of the target object. With the present invention, in such a case, rather than simply smoothing the momentary position data to thereby obtain final lateral position data, smoothing is applied to the extreme values of the momentary position data. Preferably, smoothing for deriving the corrected position data is performed by calculating a series of envelope curve line values of the rightward local extreme values (referred to in the following description as maximum values, for brevity of description) and a series of envelope curve line values of the leftward local extreme values (referred to as minimum values). If the vehicles are travelling along a straight path, as in the example of FIG. 18A, then a series of corrected position values can then be obtained as the averages of successive (rightward, leftward) concurrent pairs of envelope curve line values. These corrected position values are then smoothed, to obtain final lateral position data which will not contain large fluctuations in value, in spite of a high degree of scattering of the momentary position data (i.e., whereby there may be a very irregular distribution of the extreme rightward and extreme leftward momentary lateral position values along the time axis).
Specifically, the degree of scattering of the momentary position data is judged, and when that exceeds a predetermined first threshold value then the corrected position data are derived and smoothed, instead of the momentary position data, to obtain the final lateral position data.
Hence, the final lateral position data obtained for a target object such as a preceding vehicle can be used to obtain a more reliable estimate of the degree of probability that the object is located in the vehicle lane of the host vehicle. Thus, when such probability values are used by a vehicle control apparatus for controlling the host vehicle, increased reliability and safety of control can be achieved.
It would be possible to apply averaging directly between the two series of extreme values of the momentary position data, and use the resultant values as the corrected position data. However by first applying smoothing to these two series of extreme values, preferably, by deriving two (i.e., maximum, minimum) corresponding series of envelope curve line values), sudden changes are suppressed, so that corrected position data can be obtained which are stable and free from the effects of noise.
Furthermore, it would be possible to estimate the degree of scattering of the momentary position data based on differences between local maximum and minimum values of the momentary position data, or difference between maximum and minimum envelope curve line values derived from the extreme values. However the degree of scattering is preferably obtained as a statistical calculation of the dispersion value of a fixed number of the most recent successively obtained momentary position values.
With such a vehicle-mounted radar apparatus, when two preceding vehicles are positioned side by side, it is generally impossible to recognized these as separate vehicles, based on the received reflected waves. Thus with the two vehicles detected as a single object, the apparatus may derive a location midway between the two vehicles as being the lateral position of the xe2x80x9cobjectxe2x80x9d. In addition, the degree of scattering of the momentary position data will be extremely high. Hence, if one of these preceding vehicles is located in the vehicle lane of the host vehicle, that condition will not be accurately detected.
However with the present invention, if the degree of scattering of the momentary position data exceeds a second predetermined threshold value which is greater than the aforementioned first threshold value (i.e., if the degree of scattering substantially exceeds that which would be expected to occur for a single large-size preceding vehicle) then the final lateral position data are derived by directly smoothing the momentary position data. As a result, since in that case the strongest reflections will occur from the preceding vehicle which is in the vehicle lane of the host vehicle (i.e., is closest to the host vehicle), the final lateral position data will be biased towards the width-center location of that preceding vehicle. Hence, greater accuracy of detection can be achieved in such a case, i.e., an appropriate value of in-lane probability factor, representing the degree of probability that a preceding vehicle is located in the vehicle lane of the host vehicle, can be established for the pair of preceding vehicles.
According to another aspect, the invention provides a vehicle control apparatus comprising means for determining that a target object is likely to be a preceding vehicle, and reflection condition judgement means, operating when the target object is selected as being a preceding vehicle, for judging whether the momentary position data are affected by side reflection radio waves which are reflected from a side face of an inner side of the preceding vehicle. With such an apparatus, the corrected position data generating means comprises means for applying weighting coefficients to selectively apply weighting to the series of maximum and series of minimum envelope curve line values, before these are averaged to obtain the aforementioned corrected position data, and weighting coefficient modification means operating when it is judged that the momentary position data are affected by the side reflection radio waves, for modifying the weighting coefficients such as to apply greater weighting to a selected one of the aforementioned series of maximum values and series of minimum values. The selected series corresponds to locations on the preceding vehicle at an outer side, opposite the inner side.
In such a case in which side reflection waves are received by the radar apparatus from a preceding vehicle, the degree of scattering of the momentary position data will generally be sufficiently high that (as described above) the corrected position data will be selected to be smoothed for deriving the final lateral position data. However with the above aspect of the invention, compensation is applied to the corrected position data against the deviation which would otherwise occur due to the effects of the side face reflections upon the momentary position data.
The xe2x80x9cinnerxe2x80x9d side of a preceding vehicle can simply be determined as the side which is closest to the host vehicle, i.e., when the preceding vehicle is running in an adjacent vehicle lane, or is located in the same vehicle lane as the host vehicle, but with that vehicle lane being shaped with a significant degree of curvature.
The weighting coefficients are preferably selectively determined not only in accordance with the side of the preceding vehicle from which side face reflections are likely to be occurring, but also the relative positions/orientations of the host vehicle and preceding vehicle. That is to say, respectively different weighting coefficients may be utilized, depending upon whether the preceding vehicle is located in an adjacent vehicle lane, or is in the same lane as the host vehicle with that lane having a significant degree of curvature.
According to another aspect, the aforementioned momentary position data generating means comprises normalization means for performing normalization calculation processing to convert each of the successively obtained momentary position values to respectively corresponding normalized momentary position values. Each of these normalized momentary position values corresponds to a condition of the vehicle lane(s) being oriented along a straight line which is parallel to the (momentary) travel direction of the host vehicle.
Specifically, each momentary (lateral) position which is estimated for a target object, based on the received radar signals, is shifted laterally by an amount which is determined based on the radius of curvature of the vehicle lane of the host vehicle and the relative distance and direction of the target object. By applying such normalization to the momentary position data, all subsequent processing, including derivation of the final lateral position data, can be executed as if the host vehicle and preceding vehicles were always moving along a completely straight route. Since it is not necessary to apply compensation for the degree of curvature of the travel path each time that new lateral position data are derived, the processing is thereby substantially simplified.
According to another aspect, the apparatus includes means for estimating the width of a target object, as the width of scattering of the momentary position data. This is advantageous for the following reasons. The final lateral position data express only the successive lateral positions of the width-center location of a target object. However if the width of a preceding vehicle can be derived, then for example it becomes possible to judge the occurrence of a condition in which a preceding vehicle which is driving in a vehicle lane adjacent to that of the host vehicle is of such a size that it may be partially protruding into the lane of the host vehicle.
It would be possible for the scattering width to be obtained simply as a difference between a pair of extreme (maximum, minimum) values of the momentary position data. However with the present invention, the width of scattering is preferably obtained as an average amount of difference between aforementioned maximum envelope curve line values and minimum envelope curve line values.
According to another aspect, the apparatus includes means for assigning a target object to one of a plurality of predetermined size categories, based on the estimated width of the target object, for example a xe2x80x9cnormal-size vehiclexe2x80x9d category, xe2x80x9clarge-size vehiclexe2x80x9d category, xe2x80x9cmotor cyclexe2x80x9d category, etc. When such information is supplied to a vehicle control apparatus of the host vehicle, then more effective countermeasures against collision can be implemented, based on the estimated size of the target object, e.g., measures such as multi-stage control of opening of an air bag, etc.
Furthermore when such a vehicle-mounted radar apparatus is used in conjunction with a vehicle control apparatus such as a cruise control system, the radar apparatus can transmit, to the cruise control system, information indicating when a degree of scattering of the momentary position data for a target object is excessively high. In that case the cruise control system can be advantageously configured such that when a target degree of acceleration is established for controlling the host vehicle in relation to that target object, the target degree of acceleration is reduced if the degree of scattering of the momentary position data is excessive. This ensures increased safety of control, since a high degree of scattering of the momentary position data may indicate a low level of reliability of the position information which has been derived for the target object.
Each of the various means used to perform the above functions of a vehicle-mounted radar apparatus according to the present invention of the invention, other than radio wave transmitting and receiving functions, are preferably implemented by a program executed by a computer, for example as respective subroutines of a main program routine which is periodically executed by the computer.
Such a program could be stored on various types of data storage medium which can be read by a computer, such as floppy disk, MO disk, DVD, CD-ROM, computer hard disk, memory card, etc., such that the program can be read out from the storage medium and loaded into the computer when required, to be executed. Alternatively, the program could be stored in a ROM or in a backup RAM of the computer. Furthermore the invention is not limited to the case of the program being stored in a storage medium, and the program could for example be transmitted to the computer via a data transfer network and loaded into the computer.